A technology of detecting a vehicle traveling in the vicinity around a host vehicle and notifies a user of a result of the detection is disclosed in Japanese Patent Application Publication No. 2005-115637 (JP-A-2005-115637). In this technology, besides the forward vehicle information acquired by a radar device mounted in the host vehicle, the vehicle information acquired by a radar device mounted in another vehicle traveling in the neighborhood of the host vehicle and position information of the another vehicle are acquired via a vehicle-to-vehicle communication instrument, and the vehicle position is specifically determined by mapping the vehicle position shown by the forward vehicle information and by the communication vehicle information acquire via the vehicle-to-vehicle communication instrument.
The foregoing technology cannot cover all the directions merely by using the radar device mounted in the host vehicle, and therefore also uses the communication vehicle information acquired via the vehicle-to-vehicle communication instrument to auxiliary acquire information about regions outside the detectable regions of the radar device of the host vehicle. However, the radar device mounted in the another vehicle also has undetectable regions. Besides, it is not necessarily the case that all the vehicles traveling in the vicinity of the host vehicle are equipped with radar devices and vehicle-to-vehicle communication instruments that send communication vehicle information. Therefore, according to the foregoing technology, there is a possibility of a not-detected vehicle being present in the neighborhood of the host vehicle.